Field of the Invention
The present invention relates to a method for the relative pressure measurement using a capacitive pressure measuring cell, as well as a relative pressure measuring cell.
Background of the Invention
Various relative pressure sensors are known from prior art. A relative pressure sensor serves to measure the pressure difference between the pressure in a medium to be measured and the presently given atmospheric pressure. Such a relative pressure sensor is made from a basic body, which together with a measuring membrane connected thereto at the edge forms a sensor chamber and/or pressure chamber. For the purpose of measuring the relative pressure the reference air is guided via a ventilation opening at the side of the basic body into the sensor chamber, with here the surface of the measuring membrane facing away from the sensor chamber being impinged with the pressure to be measured. The deformation of the measuring membrane caused thereby represents a measure for the relative pressure which is converted into a measuring signal.
Ceramic capacitive pressure measuring cells, comprising a basic body sintered body and a membrane sintered body, are soldered or hard soldered to each other at their edges using a spacer such that a sensor chamber develops. The solder or active hard solder used, e.g., glass frit, serves here as the spacer itself.
Before assembling the basic body and the membrane body, here basic body electrodes and a membrane electrode are applied on their surfaces, which form the chamber walls of the sensor chamber, usually by way of sputtering tantalum or a thick-layer gold serigraphy.
When reference air is fed via the ventilation opening, moisture enters the sensor chamber, which condenses here when the dew point is fallen short, and this way it can compromise the function. As a result of an accumulation of water, the dielectric constant of the effective electrode surfaces is compromised, causing the measurement to become falsified.
In order to improve the tolerance to humidity it is suggested according to EP 1 061 351 A1 to completely coat the interior surfaces of the sensor chamber of such a pressure measuring cell with a hydrophobic material, with silane preferably being used. Due to the fact that such coatings comprising silanes or silane compounds are organic in nature, their useful temperature range is limited. Another disadvantage is given here in that such coatings, due to their temperature limitations, can only be generated after the bonding of the membrane and the basic body using a vacuum generated in the basic body using the ventilation opening, i.e. here high technical production expenses are required.
Further, a pressure measuring cell is known from EP 2 463 635 A1 in which a simplified production is yielded such that before the bonding of the membrane and the basic body a hydrophobic glass coating is applied over the entire surfaces of the basic body and the membrane previously provided with electrodes.
Due to the fact that with the above-described design approaches it is not possible to completely prevent any condensation inside the measuring chamber, still faulty measurements occur caused by condensation. This leads to the objective of the present invention. This objective is attained in a method for determining an output value as described herein as well as a relative pressure sensor according to the teaching hereof.